Just to confirm something. Orberg in LLPSI uses two different adverb variants of foris, -is (f) as follows: foris (adv.) to mean 'outside the door' and foras (adv.) to mean 'out / from the door' (ut ubi Tabellarius ianitori dixit: "Prius vinci canem et sine me intrare! Noli iterum me foras in imbrem pellere!" - I do not wish to be forced out the door into the rain.

Forīs and forās mean "outdoors" in general -- note how both English and Latin use "doors" for the metaphor of being outside the house -- but yes, you've got the difference between the two. I'm guessing it derives from the usage of the locative and accusative, like Athēnīs "at Athens", Athēnās "to Athens" -- but forīs and forās aren't the locative and accusative of forēs. So maybe it's a vestige of Old Latin or whatever Latin's ancestor was.

Also note that "nōlī" is an imperative; "nōlī + infinitive" is generally translated as "do not..." -- so "Don't drive me out into the rain again!"